This invention relates generally to computer disk drives. More specifically, the invention relates to a flexible cable used in integrated lead suspension for interconnecting a read-write head to electronic circuitry in the computer hard drive and a method for making such cable.
Computer hard drives often use a flexible ribbon cable to carry signals to and from the read/write head. The cable generally comprises one or more conductors sandwiched between two layers of flexible insulating material. As the data transfer rate to and from the storage increases to the gigahertz range, the need for a durable and flexible cable carrying high frequency current with constant electrical characteristics also increases. Especially when the cable has to be twisted and bent to fit a specific physical geometry of a suspension assembly in a computer hard disk drive. Usually, the suspension assembly is made of different materials. When the flexible cable meets different materials underneath it, the characteristic impedance of the cable changes. As a result, the reflection coefficient of the flexible cable changes, creating losses and unwanted reflection at high frequency. In addition to the increase in the speed of computer""s data retrieval to and from the storage, the size of computer and hard drive are also reduced as in lap top computers. In such situation, the flexible cable needs to fit snugly with the suspension assembly more than ever before. As the flexible cable is placed on the suspension assembly to carry information to and from the hard drive, it also has to be bent around sharps turns according to the physical geometry of each type of assembly. As a result, the characteristic impedance of the cable changes and unwanted reflection along the cable increases, making impedance matching even more challenging. Moreover, in a compact hard drive environment, the temperature may be very high, and the suspension assembly moves constantly to retrieve information, shortening the lifetime of the flexible cable. Furthermore, in high frequency application, electromagnetic interference (EMI) may distort the signals that go through the cable.
One prior art solution is to electrically ground the conductors on either side of sensitive signals. This design improves the signal quality at frequencies up to a few hundred Megahertz. However, the electromagnetic (EMI) problems still persist.
One prior art solution to the EMI problem is to coat the cable with a conductive paint to prevent the electromagnetic radiation from penetrating the cable. However, if the suspension assembly moves constantly and under high temperature, the conductive paint tends to crack. Thus, the flexible cable becomes unreliable and has to be changed regularly.
In 1989, the inventor of the present application proposed a solution to the above problems. He disclosed a flexible cable comprising of a parallel conductors disposed between two flexible insulating substrates. The two flexible insulating substrates were glued together. One of the flexible insulating substrate was punched with holes along its length to expose a center conductor. A layer of aluminum was sputter deposited over the surface of the cable, making electrical contact with the center conductor. The aluminum layer and the center conductor were electrically grounded to make the flexible cable behave as a shielded coaxial cable. Although the sputter coating solution solved the cracking problem of the conductive paint coating, the sputter deposited aluminum layer did not adhere to the polymer materials used in the insulating substrates. Therefore, a need existed in the art for a flexible cable having a strongly adhering conductive coating and a method for fabricating same.
It is a principle objective of this invention is to make a flexible cable having a strongly adhering sputter deposited conductive layer. It is a further object to provide a flexible cable with a constant characteristic impedance Z0. It is an additional object to provide flexible cable that can also be used in environments with varying geometry without any unwanted reflection or changes in its electrical characteristics, or with EMI problems. It is also an object of the present invention to provide a flexible cable with a sputter deposited conductive layer that can also sustain the heat and constant motion of the suspension assembly without cracking.
According to a first embodiment of the present invention, a flexible cable comprises one or more trace conductors sandwiched between two flexible insulating substrates. An outer surface of the cable is cleaned and sputter coated with a conductive coating. The conductive coating generally comprises a thin gold layer sputter deposited on the surface of the flexible substrates and an aluminum layer sputter deposited on top of the gold layer. The gold layer acts as a seed to improve the adhesion of the aluminum coating to the material of the flexible insulating substrates. The surface of the cable is then cleaned prior to depositing the coating by washing in an alcohol, such as isopropyl alcohol. This process allows the aluminum layer to be sputtered directly to the flexible substrate. The cable may also be cleaned by a glow discharge or sputter etching process. Either or both of the substrates may include one or more holes that expose one or more trace conductors. The conductive coating may therefore make electrical contact with the trace conductors via the holes. The trace conductors and/or the conductive coating may also be electrically grounded to provide shielding against electromagnetic interference. This cable maintains all the advantages of prior art and achieves durability and flexibility. In addition, the conductive coating on the flexible cable can successfully withstand heat and the constant motion without cracking or otherwise separating from the substrates.